This invention relates to an exhaust gas recirculation system in an internal combustion engine, and more particularly to a control system for controlling the quantity of the recirculated exhaust gas in appropriate relation to the operational condition of the engine.
It is one of effective and practicable measures against formation of nitrogen oxides NOx in the operation of an internal combustion engine, especially an automotive engine, to recirculate a portion of the exhaust gas to the intake system of the engine. The recirculation of the exhaust gas which contains large amounts of carbon dioxide causes a lowering of the maximum combustion temperature in the engine and results in suppression of the formation of NOx at the combustion of an air-fuel mixture. An exhaust gas recirculation system for this purpose consists fundamentally of a recirculation passage, which branches from the exhaust line and terminates at either a certain section of the induction passage or the intake manifold, and a flow control valve mechanism for regulating the quantity of the recirculated exhaust gas.
The provision of the flow control valve mechanism to the exhaust recirculation system is considered indispensable because the recirculation of exhaust gas is generally disadvantageous to the performance of the engine and stability of the engine operation at a variable extent depending on the operational condition of the engine. The extent of the disadvantageousness is greatest when the engine is either at idle or in operation at relatively low engine temperatures.
In a typical example of conventional flow control valve mechanisms in the exhaust recirculation system, a flexible diaphragm is arranged to govern the position of a valve member for controlling the effective cross-sectional area of the recirculation passage in response to the magnitude of vacuum created at a certain section of the engine intake system. To relate the exhaust recirculation to the engine temperature, usually an on-off functioning electromagnetic valve is utilized in combination with a temperature sensor so that the recirculation passage may be blocked when the enginge temperature is below a predetermined temperature.
In reality, both the influence of the exhaust recirculation on the efficiency and stability of the engine and the quantity of NOx produced in the engine greatly depend on numerous variables which are related to the operational condition of the engine. Examples of such variables are the rate of acceleration or deceleration, engine temperature, engine speed, ignition timing and air/fuel ratio of a combustible mixture fed to the engine. The quantity of the recirculated exhaust gas in conventional exhaust recirculation systems is not controlled in direct relation to these variables, so that the engine operation is unfavorably influenced by the exhaust recirculation to an extent by far the greater than a minimal or tolerable one. Besides, a mechanical valve-actuating means such as a vacuum-responsive diaphragm does not always make a very quick response to a change in the engine operational condition.